1. Field of the Invention
The present invention relates to an organic electro luminescence device, and more particularly, to a top emission type organic electro luminescence device and a fabrication method thereof.
2. Description of the Related Art
In the fields of flat panel display devices, a liquid crystal display device (LCD) is widely used because it is lightweight and has low power consumption. However, the LCD is a non-luminous display device and has technical limitations in brightness, contrast, viewing angle, and large size. Therefore, new flat panel display devices capable of overcoming these drawbacks have been developed actively.
One of the new flat panel display devices is an organic electro luminescence device. Since the organic electro luminescence device is a self-luminous display device, it has a high contrast and wide viewing angle compared with the LCD. Also, since the organic electro luminescence device does not require a backlight assembly, it is lightweight and slim. In addition, the organic electro luminescence device can decrease power consumption.
Further, the organic electro luminescence device can be driven at a low DC voltage and has a fast response speed. Since all of the components of the organic electro luminescence device are formed of solid materials, it is endurable against external impact. It can also be used in a wide temperature range and can be manufactured at a low cost.
Specifically, the organic electro luminescence device is easily fabricated through a deposition process and an encapsulation process. Therefore, the fabrication method and apparatus of the organic electro luminescence device are simpler than those of an LCD or PDP.
If the organic electro luminescence device is driven in an active matrix type, uniform brightness can be obtained even when a low current is applied. Accordingly, the organic electro luminescence device has advantages of low power consumption, high definition and large-sized screen.
FIG. 1 is a schematic sectional view of a related art bottom emission type organic electro luminescence device.
As shown in FIG. 1, first and second substrates 10 and 30 are arranged to face each other. Edge portions of the first and second substrates 10 and 30 are encapsulated by a seal pattern 40. A TFT T is formed on a transparent substrate 1 of the first substrate 10 in each sub-pixel unit. A first electrode 12 is connected to the TFT T. An organic electro luminescent layer 14 is formed on the TFTs and the first electrode 12 and is arranged corresponding to the first electrode 12. The organic electro luminescent layer 14 contains light emission materials taking on red, green and blue colors. A second electrode 16 is formed on the organic electro luminescent layer 14.
The first and second electrodes 12 and 16 function to apply an electric field to the organic electro luminescent layer 14.
Due to the seal pattern 40, the second electrode 16 and the second substrate 30 are spaced apart from each other by a predetermined distance. Therefore, an absorbent (not shown) and a translucent tape (not shown) may be further provided in an inner surface of the second substrate 30. The absorbent absorbs moisture introduced from an exterior, and the translucent tape adheres the absorbent to the second substrate 30.
In the bottom emission type structure, the first electrode 12 and the second electrode 16 are an anode and a cathode, respectively.
The first electrode 12 is formed of a transparent conductive material and the second electrode 16 is formed of a metal having a low work function. In such a condition, the organic electro luminescent layer 14 includes a hole injection layer 14a, a hole transporting layer 14b, an emission layer 14c, and an electron transporting layer 14d, which are sequentially formed on the first electrode 12.
Preferably, the first electrode 12 is formed of indium tin oxide (ITO) and the second electrode 16 is formed of Al, Mg or Ca, which is a metal having a low work function.
The emission layer 14c has red, green and blue color filters in sub-pixels.
However, the related art bottom emission type organic electro luminescence device has a limitation in aperture ratio and thus has difficulty in the application to high-resolution products.
FIG. 2 is a schematic sectional view of a related art top emission type organic electro luminescence device.
Referring to FIG. 2, the emission direction of light emitted from an organic electro luminescent layer 24 is opposite to that of the bottom emission type organic electro luminescence device shown in FIG. 1. For this purpose, the construction of the first and second electrodes 22 and 26 is changed.
Also, in the top emission type organic electro luminescence device, a polarization film 29 is attached on the second substrate 30 so as to solve the reduction of contrast ratio (CR), which is caused by the reflection of an external light.
For example, when the first and second electrodes 22 and 26 are respectively the anode and the cathode, the first electrode 22 must reflect light generated from an organic electro luminescent layer 24 and the second electrode 26 must transmit the light generated from the organic electro luminescent layer 24.
Accordingly, when the first electrode 22 is formed of ITO, a reflection plate 28 must be further provided below the first electrode 22 and the second electrode 26 must be formed thinly so that light can be transmitted.
In another example, when the first and second electrodes are respectively the cathode and the anode, that is, when the polarity of the electrodes in the bottom emission type are reversed, the top emission type can be provided by changing the construction of the organic electro luminescent layer formed between the first electrode and the second electrode.
Like this, in the top emission type organic electro luminescence device, the light generated from the organic electro luminescent layer 24 and passing through the second electrode 26, and the light reflected from the first electrode 22 and passing through the second electrode 26 are emitted toward the second substrate 30, thereby achieving the top emission.
In this case, however, the wavelength of the light reflected from the first electrode 22 and passing through the second electrode 26 is lengthened due to reflection, thus causing a color shift problem. The color shift problem becomes serious as the reflectivity of the first electrode 22 increases.
Also, since the polarization film 29 is attached, only the left-polarized or right-polarized light is transmitted so that optical efficiency of the organic electro luminescence device is reduced by 50%.